The present invention is directed to a method for manufacturing dimensionally accurate structures in a layer with a high aspect ratio and with dimensions in a range of 1 .mu.m and below. The method is particularly useful for the production of a mask for microelectronics. The method includes applying an auxiliary layer consisting of a metal or metal oxide on the layer to be structured and providing a lacquer layer on the auxiliary layer forming a mask in the lacquer layer, etching the auxiliary layer to form a mask in the auxiliary layer, and then subsequently etching the layer to be structured utilizing the mask formed in the auxiliary layer.
Ion beam etching processes are utilized in the manufacturing of dimensionally accurate sub-micron structures and structures having a high aspect ratio. A high aspect ratio is obtained when the height of the gap is substantially greater than the width of the gap. In these processes, the layer which is to be structured is either directly covered by a lacquer layer which is sensitive to radiation from a group consisting of photons, electrons, X-rays or ions or has an auxiliary layer interposed between the lacquer layer and this layer to be structured. The auxiliary layer is selected of a material consisting of metals, metal oxides and metal organic compounds.
The structurability of the auxiliary layer is very problematical because it is extremely dependent on the process sequence in the technology and is difficult to be reproduced. Boundary layers that have a very negative effect on the selectivity in the etching process are formed namely at exposed surfaces. Further details of the formation of boundary layers is discussed in an article by G. Horz et al, "The Role of Surface Phenomena in the Interactions of Reactive Metals with Gases", Vacuum, Vol. 33, No. 5 (1983), pages 265-270.
It is known from an article by M. Cantagrel, "Considerations of High Resolution Patterns Engraved by Ion Etching", IEEE Transactions on Electron Devices, Vol. ED-22, No. 7, July 1975, pages 483-486 to apply a thin metal film consisting of, for example, titanium, vanadium, manganese or aluminum over the layer to be etched and to structure this metal film by utilizing a lacquer mask. This structured metal film itself then serves as a mask for the layer therebelow. The fact is thereby exploited that in the case of many metals, the etching rate of the oxide with argon ions is significantly lower than the etching rate with argon ions of the metal itself. In this manner, the corresponding metal can first be structured by means of, for example, ion beam etching with pure argon and then be further employed as a mask when the etching is continued with a mixture consisting of argon and oxygen. The metal oxide is then formed at the surface of the metal mask during the etching.
However, this method has the disadvantages that the metal layer intended for use as a mask often becomes coated with an oxide layer before the structuring, for example, during storage of the substrates as well as upon application and heating of the lacquer layer. This prior oxide coating complicates the etching process. The selectivity relative to the employed lacquer layer particularly when utilizing a PMMA (polymethylmethacrylate) is too low in order to avoid a lacquer shrink. This leads to the loss of dimension and, thus, to oblique and undefined edges in the auxiliary layer which in turn cannot be transferred to the structured layer with dimensional accuracy.